The .38 S&W is a revolvercartridge developed by Smith & Wesson in 1877. It was modified for use by the British military and called the .38/200 (also known as .380" Revolver Mk IIz) in 1922 for .38 caliber pistols and revolvers which replaced the larger .455 and .476 inch handguns. It fires a .361 caliber bullet with a pressure limit of 13,000 CUP[1] It should not be confused with the .38 S&W Special cartridge which has different case dimensions and a different caliberbullet. Colt also chambered revolvers for the cartridge which they called the .38 Colt New Police - the principal difference being that the Colt rounds were flat nosed, while the S&W used a 148 grain (9.6 g) rounded nose lead bullet.

Both the Colt and the S&W cartridge claimed a muzzle velocity of 730 ft/s (220 m/s) and a muzzle energy of 176 ft·lbf (239 J).

The .38 Super Police or .38/200 version of the bullet uses a larger 200 grain (13 g) bullet with a muzzle velocity of 620 ft/s (189 m/s) and a muzzle energy of 176 ft·lbf (239 J).

After the First World War, there was a move away from the larger .455 caliber. The professional core of the pre-war British Army had been decimated and replaced by a larger and mostly conscripted force. It was recognized that the short training period available to the new recruits did not give them time to become proficient with the large-bore .455 revolvers, and that a smaller caliber would be easier for new recruits to develop competence with in pistol shooting.

Webley demonstrated a lighter version of their Mk III revolver with modified .38 S&W ammunition, firing a heavy 200 grain bullet. It received favorable reports from the Army and the revolver was accepted in principle, assuming an effective round was provided for it.

As Webley had used the .38 S&W cartridge dimensions for their revolver, and the cartridge length was fixed by the size of the cylinder of the revolver (the same as for the wider .455), Kynoch produced a cartridge with the same dimensions as the .38 S&W but with 2.8 grains (0.18 g) of "Neonite" nitrocellulose powder and a 200 grain (13.0 g) bullet. This combination gave a velocity of over 570 feet (170 m) per second at 50 yards, although the bullet become unstable after penetrating the target. This was deemed satisfactory and the design for the .38/200 cartridge was accepted into Commonwealth Service as "Cartridge, Pistol, .380" Mk IIz", firing a 180 gr (11.7 g) full metal jacket round; after it was realized that the 200 gr lead round could arguably contravene the Hague Convention, which outlawed the use of bullets designed so as to "expand or flatten easily in the human body".

The cartridge was finally phased out of Commonwealth Service in 1963, when the 9 mm Browning Hi-Power was finally issued to most units.

The Cartridge, Pistol, .380" Mk IIz is still produced by the Ordnance Factory Board in India, for use in revolvers of the Indian Army and many African countries.

Revolvers chambered for .38/200 may also fire .38 S&W (AKA .38/145), .38 Police Positive, and .38 Banker's Special cartridges, along with the .380" Mk IIz round, though caution should be exercised as always when using ammunition designed for more modern guns to different specifications.